1. Field of the Invention
This invention is related to fiber optic cable, and more specifically to techniques for providing a fiber optic cable having a desired amount of path-average dispersion.
2. Background Art
In the art of fiber optic communications, there is presently a considerable demand for providing a length of fiber optic cable having a specified amount of path-average dispersion. Such a cable may be fabricated using fiber spans containing alternating segments of positive and negative chromatic dispersion, represented by the symbol D. The instantaneous, or local, value of D at one end of the cable, denoted as D.sub.loc, is selected to be just large enough to effectively thwart the four-wave mixing products of optical signals. Typical values of D.sub.loc are in the range of around 2 to 4 picoseconds per nanometer-kilometer. However, the exact value of D.sub.loc is not especially critical, as it is more important that the path-average dispersion for the fiber span conform to a certain desired value, typically much less than D.sub.loc. The path-average dispersion of a fiber span of length L may be mathematically denoted as: ##EQU1##
For applications involving the transmission of non-return-to-zero (NRZ) data, the desired D is zero, while, for soliton data transmission, the desired D is in the range of about 0.05 to 0.5 picoseconds per nanometer-kilometer.
Using state-of-the-art technology, the fabrication of a fiber optic cable length having a desired amount of path-average dispersion is an arduous task. The chromatic dispersion of a first, negative-dispersion spool of fiber optic cable is measured, and the dispersion of a second, positive-dispersion spool is measured. Based upon the total desired length of the fiber optic cable to be manufactured, and the desired path-average dispersion of this cable, the amount of cable to be cut from the first spool is determined, as is the amount of cable to be cut from the second spool. The section of cable cut from the first spool is joined to the section of cable cut from the second spool, theoretically providing a desired length of cable that has a desired amount of dispersion.